Multimode cardiac paces with p-wave and r-wave sensing means

ABSTRACT

A cardiac pacer including a pulse generator having an R-C circuit for controlling the generation of stimulating pulses. An electrode surgically placed on or in the ventricle of the heart is coupled to the output of the pulse generator. A reference or indifferent electrode is subcutaneously implanted and coupled to the pacer circuits. When no cardiac electrical activity exists stimulating pulses are provided at a preset rate. The ventricular electrode is coupled to the input of a first signal responsive circuit and to the pulse generator. In response to ventricular electrical signals of either polarity, for example R-waves, the first signal responsive circuit resets the R-C circuit in the pulse generator to a predetermined level inhibiting the subsequent generation of impulses for a suitable time such as one second. Another electrode surgically placed on or in the atrium of the heart is coupled to the input of a second signal responsive circuit which includes timing means and which is connected in controlling relation to the pulse generator. In response to atrial electrical signals, known as P-waves, and in the absence of ventricular electrical signals, the second signal responsive circuit causes the pulse generator to provide timedelayed stimulating pulses synchronized with the atrial beats.

United States Patent Greathatch [54] MULTIMODE CARDIAC PACES WITH P-WAVEAND R-WAVE SENSING MEANS [72] Inventor: Wilson Greatbatch, Clarence, NY.

[73] Assignee: Medtronic, Inc., Minneapolis, Minn.

[22] Filed: July 16, 1969 [21] App]. No.: 842,290

[52] [1.8. CI. ..l28/4l9 P [51] Int. Cl. ..A6lll 1/36 [58] Fieldofsearch ..128/4l9 P, 421,422

[56] References Cited UNITED STATES PATENTS 3,433,228 3/1969 Keller, Jr...l28/4l9 P 3,253,596 5/1966 Keller, Jr. ..l28/4l9 P 3 ,528,428 9/ l 970Berkovits 128/419 P FOREIGN PATENTS OR APPLICATIONS 1,082,752 9/1967Great Britain ..l28/4l9 P OTHER PUBLICATIONS F ischler et al. Instituteof Electric and Electronic Engineers Transactions on BioMedica]Engineering Vol. BME 16, No. l,lan. I969 1 Mar. 14, E972 Castillo et al.Chest" Vol. 59, No. 4, Apr. 197i pp. 360- 364 Primary ExaminerWilliam E.Kamm AttorneyLew Schwartz and Donald R. Stone [5 7] ABSTRACT A cardiacpacer including a pulse generator having an R-C circuit for controllingthe generation of stimulating pulses. An electrode surgically placed onor in the ventricle of the heart is coupled to the output of the pulsegenerator. A reference or indifierent electrode is subcutaneouslyimplanted and coupled to the pacer circuits. When no cardiac electricalactivity exists stimulating pulses are provided at a preset rate. Theventricular electrode is coupled to the input of a first signalresponsive circuit and to the pulse generator. In response toventricular electrical signals of either polarity, for example R-waves,the first signal responsive circuit resets the R-C circuit in the pulsegenerator to a predetermined level inhibiting the subsequent generationof impulses for a suitable time such as one second. Another electrodesurgically placed on or in the atrium of the heart is coupled to theinput of a second signal responsive circuit which includes timing meansand which is connected in controlling relation to the pulse generator.In response to atrial electrical signals, known as P-waves, and in theabsence of ventricular electrical signals, the second signal responsivecircuit causes the pulse generator to provide time-delayed stimulatingpulses synchronized with the atrial beats.

17 Claims, 5 Drawing Figures DETiC TOP P"WAVE DETECTOR HVHIBIT PULSEPatented March 14, 1972 3,648,707

3 Sheets-Sheet l 0N6 HEART BEAT I R- WA V5 10 DE 75c TOR 22 INHIBIT i3PU; s5 GEM 25 K P- WA v5 DETECTOR l I 77l- 17 INVENTOR.

WILSON 625/9 73/; TC/-/ /g/-LdiZZ$ @w/w A T TOR/W1" KS" Patented March14, 1972 3,648,707

S Sheets-Sheet 2 INVENTOR.

WILSON GREHTBHTCH gWM ATTORNEYS" Patented March 14, 1972 3,648,707

5 Sheets-Sheet 5 WILSON GREHTBATC/l K INVENTOR.

ATTORNEYS MULTIMDDE CARDIAC FACES Wll'llllil lP-WAVIE AND lit- WAVIESENSING MEANS BACKGROUND OF THE INVENTION This invention relates toelectronic cardiac pacer and, more particularly, to a cardiac pacerwhich may or may not be implantable within the human body and which willrespond to the changing needs of the body but will not compete with thenatural cardiac electrical activity of any kind.

Electronic cardiac pacemaking began in 1952 with a device demonstratedby Dr. 2011 which was capable of passing a stimulating impulse strongenough to elicit a heart beat through the patients chest. The Zolldevice was limited to short-term applications since the current levelsrequired were of such magnitude that stimulation was accompanied bysevere pain and burning of the skin at the electrode site.

The implantable cardiac pacer, shown in US. Pat. No. 3,057,356 permitsinnocuous, painless, long-term cardiac stimulation at low power levelsby utilizing a small, completely implanted transistorized andbattery-operated pacer connected via flexible electrode wires directlyto the myocardium or heart muscle. Such a nonsynchronous pacer, whileproviding fixed-rate stimulation not automatically changed in accordancewith the bodys needs, has proven effective in alleviating the symptomsof complete heart block. A nonsynchronous pacer, however, has thepossible disadvantage of competing with the natural, physiological pacerduring episodes of normal sinus conduction.

As a result there has developed a demand-type pacer, in which case theartificial stimuli are initiated only when required and subsequently canbe eliminated when the heart returns to the sinus rhythm. Such a demandpacer is shown in my pending application Ser. No. 455,132, filed May 5,1965, entitled CARDIAC IMPLANTABLE PACER," now US. Pat. No. 3,478,746,issued Nov. 1969. The demand pacer solves the problem arising innonsynchronous pacer by inhibiting itself in the presence of ventricularactivity but coming on line and filling in missed heart beats in theabsence of ventricular activity. When the demand pacer comes on line, itoperates as a nonsynchronous pacer, the rate of which is not responsiveto atrial activity.

A synchronous or P-wave pacer has been proposed for producing a stimulusfollowing each P-wave or atrial beat. When the body signals a need forincreased heart rate, as indicated by an increasing atrial beat, thesynchronous pacer responds with an increased ventricular stimulationrate. However, the function of proposed synchronous pacer is notresponsive to irregular ventricular ectopic activity and may competeagainst such beats. Thus, while the synchronous pacer will not competeagainst normally conducted beats, it can compete against ectopic orabnormally conducted beats. Any competition between the natural and theartificial pacer may be undesirable because such competition maypossibly lead to incidents of tachycardia or even fibrillation.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto provide an artificial cardiac pacer which is responsive to thechanging needs of the human body yet will not compete under anycircumstances with natural cardiac electrical activity, normallyconducted or ectopic.

It is a more particular object of this invention to provide anartificial pacer which stimulates the heart nonsynchronously in theabsence of cardiac electrical activity of any kind, inhibits itself andbecomes completely dormant for a suitable interval in the presence of asingle ventricular beat, ectopic or conducted, and stimulates the heartsynchronously in the presence of atrial activity not accompanied byarrythmic ventricular activity.

It is a further object of this invention to provide such an an tificialpacer which can be implanted and which is automatically responsive tocardiac electrical signals, such as R-wave and F'waves, of eitherpolarity.

It is a further object of this invention to provide such an artificial,implantable pacer, the operation of which will not be influenced orimpaired by environmental electrical signals such as conventional60-cycle alternating current.

The present invention provides an artificial cardiac pacer which, in theabsence of natural cardiac electrical activity of any kind, providesstimulating electrical pulses to the heart at a predetermined rate. Inresponse to a ventricular beat, either ectopic or conducted and from anysource, the pacer inhibits itself and becomes completely dormant for asuitable interval. In response to atrial electrical activity, notaccompanied by natural ventricular activity, the pacer providesstimulating pulses suitably delayed and in synchronism with the atrialbeats.

The foregoing and additional advantages and characterizing features ofthe present invention will become clearly apparent upon reading of theensuing detailed description of an il|ustrative embodiment thereoftogether with the included drawings depicting the same,

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the voltage waveproduced by a human heart during one complete heart beat;

FIG. 2 is a block diagram illustrating a preferred embodiment of anartificial pacer according to the present invention;

FIG. 3 is a schematic diagram of the pulse generating circuit includedin the system of FIG. 2;

FIG. d is a schematic diagram of the first signal responsive circuitincluded in the system of FIG. 2; and

FIG. 5 is a schematic diagram of the second signal responsive circuitincluded in the system of FIG. 2.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT The human heart beatis represented electrically as a complex wave consisting of what aredesignated F," Q, "It, 5" and T waves all as shown in FIG. ll. TheP-wave electrically represents an atrial beat associated with atrialdepolarization which beat commands heart rate as a function of signalsfrom the rest of the body. The major and most pronounced electricalpulse in the heart is the R-wave and is normally of a magnitude between2 to 20 millivolts in the ventricle. The R-wave, which stimulatesventricular contraction, typically has an amplitude relation to theP-wave of at least three to one. The R-wave normally is generated bydepolarization of the ventricles, but when not so produced due to somecardiac malfunction, it is the function of the artificial pacer toprovide periodic electronic pulses to the heart to supply a missingR-wave. If both the natural and artificial pacer supply an lR-wave,however, competition for control of the heart results and a possiblydangerous situation arises when the pacer electronic pulse occurs in aT-wave region. The T-wave portion of each complete heart beat followsthe R-wave or major beat pulse thereof by about 0.3 seconds. Within theT- wave is a critical interval known as the vulnerable period and, inthe case of a highly abnormal heart, a pacer impulse falling into thisperiod can conceivably elicit bursts of tachycardia or fibrillationwhich are undesirable and may even lead to fatal sequence of arrythmias.

FIG. 2 shows in block diagram form an artificial cardiac pacerconstructed in accordance with the present invention. The pacer includespulse generating means It] for providing stimulating pulses to the heartunder certain conditions and at a preset rate controlled by timing meansincluded therein. A first electrode ii is coupled to pulse generatingmeans Iii and is adapted to be operatively connected to a patients hearton or in the ventricle thereof. A second electrode 112, which functionsas a reference or indifferent electrode, can be placed in contact withanother portion of the patients body or even with a selected portion ofthe patients heart. The pacer includes a third electrode 13 adapted tobe operatively connected to a patients heart on or in the atriumthereof. This electrode is coupled to another component of the pacer aswill be described presently.

The electrode arrangement shown is preferred, although others can besubstituted without departing from the spirit and scope of the presentinvention. Only one ventricular electrode 11 is needed and is placedsurgically on or in the ventricle of the heart. The single referenceelectrode 12 can be subcutaneously implanted. The electrodes cancomprise, for example, myocardial wires or plates or can be of thesalt-bridge type. In preferred form, electrodes 11 and 12 are of thetype which provide a sensing function as well as a driving function,although a separate electrode structure for sensing ventricularelectrical signals can be provided. Electrode 13 is placed surgically onor in the atrium of a patients heart.

Electrode 11 is coupled through leads 14 and 15 to pulse generatingmeans 10. Electrode 12 is coupled through a lead 16, a ground plate 17connected thereto, and a lead 18 to pulse generating means 10. Plate 17,which is included in the pacer, serves an electrical ground withelectrode 12 for pulse generating means 10 and also as a ground forother circuit components of the pacer as will be described presently.

The pacer provided by the present invention also comprises a firstsignal responsive means 19 which is responsive to ventricular electricalsignals in the heart and thus also may be called an R-wave detector.Signal responsive means 19 is coupled to ventricular electrode 11through a lead 20 and lead 14 and to electrode 12 through a lead 21which is connected to ground plate 17. Signal responsive means or R-wavedetector 19 is operatively connected through a lead 22 to pulsegenerating means 10, and a ventricular signal, such as an R- wave,produced in the heart is sensed by means 19, the signal is amplifiedtherein, and pulse generating means 10 is inhibited or recycled so thatno stimulating pulse will be sent to the heart by the artificial pacer.A detailed description of the circuitry and operation of signalresponsive means 19 will be presented further on in the specification.

The pacer additionally comprises a second signal responsive means 23which is responsive to natural atrial beats of the heart and may bedesignated a P-wave detector. Signal responsive means 23 is coupled toatrial electrode 13 by a lead 24, is operatively connected to pulsegenerating means 10 through a lead 25, and is connected to ground plate17 through line 26. Signal responsive means or P-wave detector 23includes timing means so that each natural atrial beat causes means 23to provide an amplified and time-delayed signal for activating pulsegenerating means 10 a predetermined time after the atrial beat andcoordinated therewith.

The pacer is provided with suitable power supply means such as a battery(FIG. 3) for operating pulse generating means 10 and first and secondsignal responsive means 19 and 23. In addition, the pacer of the presentinvention can be either of the external type or implanted within thehuman body. To be implanted, the entire pacer, including its battery,which can be rechargeable, is encased in an envelope of a moistureproofand human body reaction-free material such as silicone rubber orsuitable plastic so as to permit long-term implantation within the humanbody. Similar material is employed also to envelope leads 14, 16 and 24extending between the pacer circuit components and electrodes 11, 12 and13, respectively. The aforementioned US. Pat. No. 3,057,356 can bereferred to for additional information pertaining to the structuraldetails of implanted pacer.

The operation of the artificial pacer shown in FIG. 2 is as follows. Ifno atrial or ventricular electrical activity occurs, the pacer willstimulate the ventricle at a preset rate. In particular, when no naturalR-waves are sensed by ventricular electrode 11, and no P-waves aresensed by atrial electrode 13, signal responsive means 19 and 23 are ina quiescent state. Following an unavoidable inherent refractory timesuch as 100 milliseconds which follows each stimulation impulsegenerated by the pulse generating means 10, pulse generating means 10operates to provide an output of stimulating pulses which appear betweenelectrodes 11 and 12. The pulses occur at a rate determined byadjustable timing means included within generating means 10, thedetailed construction and operation of which will be described furtheron in the specification. Thus, in this mode, when cardiac electricalactivity is absent, the device functions as a nonsynchronous pacerwithout reference to any external inputs.

The R-wave stimulated in the heart by the artificial pacer will, ofcourse, be sensed by electrode 11 and activate signal responsive means19. This, in turn, results in signal generating means 10 being inhibitedfor about one second following the R-wave. The reason for this is toprevent the occurrence of stimulating impulses during the T-wave portionof the heart beat period.

In response to a ventricular beat, either ectopic or normally conductedand from any source, the pacer inhibits itself and becomes completelydormant for a suitable interval. In this mode, operation is that of ademand pacer. Should any ectopic activity occur, the pacer goes offline" and remains dormant until such time as all ventricular cardiacelectrical activity has ceased for about one second. More particularly,electrical signals indicative of ventricular activity such as conductedR- waves or ectopic signals, are picked up by electrode 11 with theresult that signal responsive means 19 is activated. In addition, signalresponsive means 19 is responsive to cardiac electrical signals ofeither polarity, due to a strong mathematical differentiation in itscircuit. Activation of means 19, in turn, provides a signal on line 22which inhibits pulse generating means 10 or, more specifically, whichcauses a recycling of generating means 10.

In response to an atrial beat without any ventricular activity of anykind, natural or ectopic, the pacer provides a ventricular stimulationimpulse following each sensed P-wave by a suitable interval. In otherwords if atrial activity is present and not accompanied by naturalventricular activity, the pacer will generate ventricular stimuli,synchronized with and delayed behind each P-wave. The pulse generatingmeans 10 contains timing means and a trigger input amplitude so selectedthat a premature P-wave will fail to trigger the pulse generating means10. This selection provides an inherent refractory period or lockout" inthe synchronized mode, as described in the next paragraph, thateffectively limits the rate at which the pacer can be driven in thesynchronous mode. In particular, the occurrence of an atrial P-wave issensed by electrode 13, and the signal is transmitted over line 24 tosignal responsive means 23, or P-wave detector, where it is amplifiedand delayed. The delayed signal is then transmitted over line 25 topulse generating means 10, whereupon a heart-stimulating pulse isgenerated and then transmitted to the heart through electrodes 11, 12.The heart stimulating pulse is properly delayed about 0.12 secondsbehind the sensed P-wave and synchronized to it.

Thus, the artificial pacer shown in FIG. 2 is responsive to the changingneeds of the body, yet will not complete, under any circumstances, withnatural cardiac electrical activity either conducted or ectopic. Somehazard could exist from permitting signal responsive means or P-wavedetector 23 to fire the pulse generator 10 at any random time. Should aP- wave occur about 180 milliseconds after a stimulated R-wave, a secondstimulated R-wave could occur one P-R interval later and might fallwithin the vulnerable zone of the T-wave. Although such an event isphysiologically unlikely, such an eventuality is eliminated by a lockoutof P-wave detector 23 for some suitable time following an R-wave. Thissame feature places a limit on the rate at which the artificial pacercan run synchronously. If two P-waves are sensed in rapid succession,only the first will activate the pulse generator 10 and the second willbe locked out. As the atrial rate rises even further only every thirdP-wave will activate pulse generator 10. A suitable lockout period wouldbe 500 milliseconds, limiting the maximum synchronous rate to beats perminute.

Thus if the atrial rate rises above 120 beats per minute, the pulsestimulation means 10 will operate in response only to every otherP-wave. If, on the other hand l second elapses with no atrial orventricular activity, the pacer will operate in the freelrunning mode.But under no conditions can either synchronous or nonsynchronous paceroperation provide stimulation impulses during the T-wave portion of theheart beat period. in addition, the use of separate electrodes placed onthe ventricle and on the atrium of the heart to sense R- waves andP-waves, respectively, advantageously utilizes the positional amplitudedifferentiation associated with the two types of heart signals. Thisplus the fact that the frequency spectrum of lR-waves and P-waves eachare accommodated only by a corresponding one of the signal responsivemeans 119 and 23, respectively, enhances the operational capability andefficiency of the artificial pacer.

The structure and operation of the various circuit components of thepacer system shown in FIG. 2 will be described now in detail. Apreferred circuit arrangement for pulse generating means is shown inFIG. 3. The circuit 110 includes two input terminals 30 and Eli adaptedto be connected to lines 22 and 25, respectively, shown in FIG. 2, andtwo output terminals 32 and 33 adapted to be connected to lines and 18,respectively. A source of electrical power in the form of battery 34 isprovided and connected between a positive supply voltage line 35 and aground potential line 36 which itself is connected to terminal 33.Although a separate battery is shown for pulse generating means it) andwill be shown for first and second signal responsive means 19 and 23,respectively, this is merely for convenience in illustration. Preferablya single battery would be included to provide electrical power for allthree circuit components in a compact arrangement, especially when thepacer is of the type to be implanted within the human body.

Pulse generating means i0 is operable on low power and includes anoscillator transistor 37 the emitter of which is connected through aresistor 35 to ground conductor 36. The base of transistor 37 isconnected through a timing capacitor to a transformer ill, in particularto one end ofa secondary winding 42 thereof. The other end of secondarywinding 42 is connected to ground conductor 36. The primary winding d3of transformer M is connected between the collector of transistor 37 andpositive supply voltage conductor 35. A transient protection diode idand current limiting resistor 455 are connected between supply voltageconductor and the collector of transistor 37 in parallel with primarywinding d3.

Pulse generating means 10 also includes an amplifier transistor ibhaving its control or base terminal connected through a resistor l7 tothe junction of timing capacitor and secondary winding 42. The collectorof transistor lb is connected through a resistor to supply voltageconductor 35. An output capacitor d9 is connected between the collectorof transistor as and terminal 32. Pulses generated by the means 10appear between terminals 32, 33 and thus between electrodes ill, i2 onebeing placed on or in the ventricle of a patients heart. The emitter oftransistor d6 is connected directly to ground conductor 35.

Signal generating means it) also includes a semiconductor switch in theform of transistor 50, the control or base terminal of which isconnected to input terminal 30. The collector of transistor is connectedto the junction of resistor 39 and the base or control terminal ofoscillator transistor 37. The emitter terminal of transistor 50 isconnected directly to ground conductor 35. Input terminal 3 is connecteddirectly to the junction of timing capacitor d0 and the base or controlterminal of oscillator transistor 37.

Pulse generating means 10 operates as follows. When no natural orectopic cardiac electrical activity occurs, no signals appear on inputterminals 3 0, 31. Current flowing from battery 34 through resistor 39charges timing capacitor 40, and at the same time output capacitor &9charges to a voltage level near that of battery 3d and electrodes illand 12 both assume ground potential. After a time predetermined by thevalues of resistor 39 and capacitor it) which comprise an lR-C timingcircuit, the voltage at the base of transistor 37 reaches a levelsufficient to forward bias the base -emitter junction thereof.

The turning on of transistor 37, in turn, provides a path for the flowof current through transformer primary winding d3, the collector-emitterpath of transistor 37, and resistor 35. its a result, a voltage isinduced in transformer secondary winding 32 which turns on transistor 46and drives transistor 37 rapidly into saturation.

When transistor do turns on, the collector thereof is at nearly groundpotential. Since the voltage across capacitor W cannot changeinstantaneously, ventricular electrode llil, being connected to terminal32, is; driven negatively by an amount nearly equal to the supplyvoltage, and capacitor W discharges through the collector-emittercircuit of transistor 46 causing a current to flow from electrode 112(which is connected to terminal 33) to electrode ill.

While transistor 37 is on, capacitor 40 discharges and rechargespartially in the opposite direction. Transformer il saturates and thefield created about primary winding 33 begins to collapse, immediatelyreversing the polarity of voltage on secondary winding 32. This polarityreversal, in turn, drives transistors 37 and 436 immediately intocutofi' which terminates the stimulating pulse provided by the pacer,preferably about 0.50 to 2.0 milliseconds after beginning of the pulse.The reverse charge on capacitor d0 maintains transistor 37 in the cutoffstate until the charge is again reversed by current flowing from battery34 through resistor 39. As transistor do turns off, capacitor 4L9 beginsto charge through resistor 48, and a slow charge current flows fromterminal 32 to terminal 33 and thus from electrode llll to elec trode i2until capacitor 35 is fully charged. A nonsymmetrical, biphasic currentflow thus results between electrodes ill and T2.

The pacer stimulation pulse rate is dependent upon the values of timingcapacitor 4-0 and resistor 39 and preferably is about one pulse persecond. The pulse rate is adjustable conveniently when resistor 35 is ofthe variable type, and the pacer can be designed so that rate adjustmentcan be per formed after implantation with percutaneous needle.

The pulse generating means 10 continues operation in this free-runningmode at a predetermined rate in the absence of signals on inputterminals 30, 31. When, however, signal generating means 19 provides anoutput on line 22, as when a natural or pacer stimulated R-wave issensed, or when ventricular ectopic signals occur, the resulting signalon terminal 30 activates transistor 50. Transistor 50 remains on longenough, for example about 20 milliseconds, to assure that timingcapacitor i0 is discharged to the proper level. in other words,transistor 50 by discharging capacitor 40 prevents the voltage on thebase terminal of transistor 37 from reaching the value needed to forwardbias the base-emitter junction thereof, such voltage level beingrequired to generate pacer pulses on electrodes llll, l2.

When, on the other hand, signal generating means 23 provides an outputon line 25, in response: to the occurrence of P- waves, the resultingsignal on terminal 3ll will turn on transistor 37 if a signal is notpresent at terminal 30. This, in turn, causes signal generating means toprovide stimulating pulses at a rate synchronized with the naturalatrial beats of the heart.

HG. t reveals a preferred circuit arrangement for signal responsivemeans if, one function of which is to sense ventricular electricalsignals of either polarity. The circuit T9 includes an input terminal 55adapted to be connected to lead 20 and an output terminal 56 adapted tobe connected to lead 22. A source of electrical power in the form ofbattery 57 is provided and connected between a positive supply voltageconductor 533 and a ground potential conductor 59. Ground conductor 59is connected to a third terminal 60 which, in turn, is connected throughline 211 to ground plate 117.

Signal responsive means 15' includes a notch-type filter 451 comprisinga first branch consisting of series-connected resistors 62, 63 connectedin parallel with series-connected capacitors 6d, es. Filter 6i alsocomprises a second branch consisting of capacitor ob connected betweenthe junction of resistors 62, 63 and ground conductor 59 and resistor 67connected between the junction of capacitors 64, 65 and ground conductor59. The voltage output of filter 61 appears across a load resistor 68which is connected between the junction of resistor 63 and capacitor 65and ground conductor 59. Filter 61 is designed to transmit all signalsexcept those having a frequency of about 60 c.p.s. Typical values forthe components thereof are as follows: resistors 62 and 63, 300k;resistor 67, l50K; capacitors 64 and 65, 0.01 u f.; capacitor 66, 0.02 nf. By rejecting signals having a frequency of around 60 c.p.s., filter61 prevents the pacer from being influenced or impaired by ordinaryhousehold electrical current, which the patient may on occasion beexposed to.

Resistor 68 is also an input resistor for a first semiconductoramplifier stage including field effect transistor 69 having gateelectrode 70 and source and drain electrodes 71 and 72, respectively.The field effect transistor is a voltage-controlled semiconductor deviceand, in addition, can operate suitably with a high input resistance, forexample in the range l-l Megohms, Gate electrode 70 of transistor 69 isconnected to the junction of resistor 68 and filter 61, and sourceterminal 71 is connected through the parallel combination of resistor 73and bypass capacitor 74 to ground conductor 59. Drain electrode 72 isconnected to the input or base terminal of a second semiconductoramplifier stage comprising transistor 75, the collector terminal ofwhich is connected to source terminal 71 of transistor 69. A currentlimiting resistor 76 is connected between supply voltage conductor 58and the emitter of transistor 75.

A third semiconductor amplifier in the form of transistor 77 isincluded, the base terminal of which is connected through a resistor 78to the emitter terminal of transistor 75. The collector terminal oftransistor 77 is connected through a resistor 79 to supply voltageconductor 58, and the emitter terminal of transistor 77 is connected toground conductor 59 through a parallel combination of a resistor 80 anda bypass capacitor 81. Transistor 77 is connected in common emitterconfiguration.

The signal responsive means also includes semiconductor switching meansin the form of transistor 83, the base terminal of which is connectedthrough a coupling capacitor 84 to the output of amplifier transistor77. The base terminal of switching transistor 83 also is connectedthrough a resistor 85 to supply voltage conductor 58, and the emitterterminal of transistor 83 is connected directly to supply conductor 58.The collector terminal of switching transistor 83 is connected through aresistor 86 and a capacitor 87 to ground conductor 59. A voltage dividerincluding resistors 88 and 89 is connected between resistor 86 andground conductor 59 and thus in parallel with capacitor 87, Outputterminal 56 of signal responsive means 19 is connected to the junctionof resistors 88 and 89.

Signal responsive means 19 operates in the following manner to senseeach natural R-wave, ectopic ventricular signals, and the signalgenerated in the heart in response to each pacer pulse. Ventricularelectrical activity, as sensed by electrode 11 which is connected byleads 14 and to input terminal 55, is indicated by a pulse of positiveor negative polarity appearing on terminal 55. The pulse is transmittedthrough filter 61 and amplified by field effect transistor 69 and byamplifier transistor 75 and mathematically differentiated by resistor 73and capacitor 74. The pulse is amplified further and furtherdifferentiated by the combination of transistor '77, resistors 79 and80, and capacitor 81. The parallel combination of resistor 80 andcapacitor 81 is still another differentiating network which further aidsin transforming the monopolar signal of either polarity into a bipolarsignal. The order in which the positive and negative going portions ofthe bipolar signal are generated depends upon the polarity of the inputpulse.

The negative going portion of the bipolar signal appearing on thecollector of transistor 77 is shaped by capacitor 84 and resistor 85,and the resulting signal turns on switching transistor 83. The turningon of transistor 83 provides a pulse on output terminal 56, which pulseis transmitted through lead 22 to the base terminal of switchingtransistor 50 included in signal generating means 10. Capacitor 87 andresistor 88 function as a pulse shaping network for this pulse whichactivates transistor 50. The network is designed to provide a pulsewidth such that transistor 50 stays on long enough, for example about 20milliseconds, to assure that timing capacitor 40 in signal generatingmeans 10 is discharged to the proper level. Thus, in response to R-wavesproduced naturally by the heart when it is beating normally, pulsegenerating means 10 is continually reset and does not providestimulating pulses to the heart.

When stimulated by a pacer pulse, the heart and the electrode contactsto it react as a complex impedance containing reactive components whichalong with delays in circuit recovery from saturation provide arefractory period of about 100-200 milliseconds after the pacer pulse.Any R-wave arriving after that time activates signal responsive means 19to turn on switching transistor 50 in pulse generating means 10. This,in turn, discharges timing capacitor 40 to reset pulse generating means10 and prevent stimulating pulse from occuring during the T-wave portionof the heart period or cycle.

If the heart should be beating normally and then skip a beat, the pacerwill insert a stimulating pulse. The R-wave from the last naturalheartbeat is sensed by signal responsive means 19 and causes a dischargeof timing capacitor 40 in pulse generating means 10. Capacitor 40 thenbegins the next charging cycle and is not discharged again. The totalelapsed time between the last natural heartbeat and the first pacerstimulating pulse is the charge time of capacitor 40 to the levelrequired to turn on oscillator transistor 37. This, of course, assumesnegligible transit time of the R-wave through the circuit of signalresponsive means 19.

Thus, when the heart is not beating naturally and successive pacerpulses are generated, the heartbeat generated by each pacer pulse isalso sensed and amplified by signal responsive means 19 and utilized toactivate transistor 50 which, in turn, discharges timing capacitor 40 toa reference level near ground potential. Timing capacitor 40 then beginscharging toward a voltage level sufficient to activate oscillatortransistor 37.

FIG. 5 reveals a preferred circuit arrangement for signal responsivemeans 23, the function of which is to sense P-waves of either polarityand in response thereto provide a delayed signal on line 25 foractivating pulse generating means 10. The circuit 23 includes an inputterminal adapted to be connected to lead 24 and an output terminal 96adapted to be connected to lead 25. A source of electrical power in theform of battery 97 is provided, and is connected between a positivesupply voltage conductor 98 and a ground potential conductor 99. Groundconductor 99 is connected to a third terminal 100 which is connected toground plate 17 through lead 26.

Signal responsive means 23 includes a filter 101 connected to inputterminal 95 for the same purpose that filter 61 is included in R-wavedetector 19, i.e., to prevent potentially dangerous interference withpacer operation by 60-cycle alternating current. Filter 101 comprises afirst branch consisting of series connected resistors 102, 103 connectedin parallel with series-connected capacitors 104, 105. Filter 101 alsocomprises a second branch consisting of capacitor 106 connected betweenthe junction of resistors 102, 103 and ground conductor 99 and resistor107 connected between the junction of capacitors 104, and groundconductor 99. Since filter 101 also is designed to reject signals havinga frequency of around 60 c.p.s., the components thereof can have thesame magnitudes as those of filter 61, previously described.

The voltage output of filter 101 appears across a load resistor 108connected between the junction of resistor 103 and capacitor 105 andground conductor 99. Resistor 108 also is an input resistor for a firstsemiconductor amplifier stage including field effect transistor 109having gate electrode 110 and source and drain electrodes 111 and 112,respectively,

Transistor 109 is included in t he circuit for the same reasons thatfield effect transistor 69 is provided in the circuit of signalresponsive means 19. Gate electrode 110 of transistor 109 is connectedto the junction of resistor 108 and filter 101, and source terminal 111is connected through the parallel combination of resistor 113 and bypasscapacitor 114 to ground conductor 99. Drain electrode 112 is connectedto the base terminal of a second semiconductor amplifier stagecomprising transistor 115, the collector terminal of which is connectedto source terminal 111 oftransistor 109. A load resistor 116 isconnected between supply voltage conductor 98 and the emitter oftransistor 115.

A third stage semiconductor amplifier in the form of transistor 117 isincluded, the base terminal of which is connected through a resistor 110to the emitter terminal of transistor 115. The collector terminal oftransistor 117 is connected through a resistor 119 to supply voltageconductor 98, and the emitter terminal of transistor 117 is connected toground conductor 99 through a parallel combination of resistor 120 and abypass capacitor 121.

The circuit arrangement thus described amplifies the input signalreceived and converts it from a monopolar signal of either polarity intoa bipolar signal by mathematical differentiation. The next stage ofsignal responsive means 23 functions to transform this bipolar signalinto a trigger pulse, and includes a switching transistor 122, the baseterminal of which is connected through a coupling capacitor 123 to thecollector terminal of transistor 117. The base terminal of switchingtransistor 122 also is connected through a resistor 124 to supplyvoltage conductor 98, and the emitter terminal of transistor 122 isconnected directly to supply conductor 98. The collector terminal ofswitching transistor 122 is con nected through a resistor 125 and acapacitor 126 to ground conductor 99. A voltage divider includingresistors 127, 128 is connected between resistor 125 and groundconductor 99 and thus in parallel with capacitor 126.

The values of capacitor 126 and resistors 127, 128 are selected suchthat repetitive signals above a certain rate such as perhaps cps. andexemplified by ignition noise, radar impulses etc., will result in acharging of capacitor 126 and a cutting off of transistor 122, thusmaking the amplifier unresponsive to fast and repetitive pulsatingsignals. Signal responsive means 19 also is provided with a similarcircuit for this purpose comprising capacitor 87 and resistors 08, 99.

The component or stage for producing a trigger pulse further includes aninverter stage comprising transistor 129, the base terminal of which isconnected to the junction of resistors 127 and 128. The collectorterminal of transistor 129 is connected through a resistor 130 to supplyvoltage conductor 98, and the emitter terminal thereof is connecteddirectly to ground conductor 99. The collector terminal of invertertransistor 129 is connected to one terminal ofa capacitor 131, the otherterminal of which is connected through a resistor 132 to groundconductor 99. Capacitor 131 and resistor 132 differentiate the pulseoutput ofinverter transistor 129 to provide a sharp trigger pulse forthe next circuit stage.

The next circuit component or stage is a timing means 133 which inresponse to the occurrence ofa trigger pulse provides an output pulsehaving a predetermined width after a predetermined time delay. Timingmeans 133 is a multivibrator-type circuit and includes a firsttransistor 134, the collector terminal of which is connected through aresistor 135 to supply voltage conductor 98. The collector terminal alsois connected to the anode ofa diode 136, the cathode ofwhich isconnected to the junction of capacitor 131 and resistor 132. Diode 136thus is connected so as to allow conduction only of negative-goingtrigger pulses to timing means 133. The emitter terminal of transistor134 is connected directly to ground conductor 99. Timing means 133includes a second transistor 137, the base terminal of which isconnected through a resistor 138 to supply voltage conductor 98. Thebase terminal of transistor 137 also is connected through a timingcapacitor 139 to the junction of resistor 135 and the collector terminalof transistor 134. The collectE t erminal of transistor 137 is connectedthrough a resistor 140 to supply voltage conductor 93 and through aresistor 141 to the base terminal of transistor 134.

The output of timing means 133 is difierentiated and inverted by a pulseshaping means whereupon a pulse output is provided on terminal 96. Thecollector terminal of transistor 137 is connected to one terminal of acapacitor 142, the other terminal of which is connected through aresistor 143 to ground conductor 99. Capacitor 142 and resistor 143comprises a differentiating network which operates on the delayed pulseoutput provided by timing means 133. A diode 144, the cathode of whichis connected to the junction of capacitor 142 and resistor 143, allowstransmission of only the negativegoing portions of the differentiatedsignal. An inverter stage comprises transistor 145, the base terminal ofwhich is connected to the anode of diode 144 and, through a resistor 151to the supply voltage conductor 98. The emitter terminal of transistor145 is connected directly to supply voltage conductor 90, and thecollector terminal thereof is connected through resistors 146 and 147 toground conductor 99. The junction of resistors 146, 147 is connected tooutput terminal 96, and a Zener diode 140 is connected from thisjunction to ground.

Signal responsive means 23 operates in the following manner to senseeach natural P-wave generated in the atrium of the heart and to providea delayed output pulse in response thereto. Atrial electrical activity,as sensed by electrode 13 which is connected by lead 24 to inputterminal 95, is indicated by a monopolar pulse of positive or negativepolarity appearing on terminal 95. The pulse is transmitted throughfilter 101 and amplified by field effect transistor 109 and by amplifiertransistor 115. The monopolar pulse is amplified further and convertedinto a bipolar signal by the combination of transistor 117, resistors119 and 120, and capacitor 121 and other elements. The differentiationis performed in a manner similar to the differentiation by thecorresponding portion of signal responsive means 19.

The negative-going portion of the bipolar signal appearing on thecollector of transistor 117 is shaped by capacitor 123 and resistor 124,and the resulting signal turns on switching transistor 122. The turningon of transistor 122 provides an output pulse which is transmitted fromresistors 127, 128 to the base terminal of transistor 129 which invertsthe pulse. The negative-going, sharp trigger pulse provided by capacitor131 and resistor 132 passes unimpeded through diode 136 and capacitor139, turning off transistor 137 which is biased so as to be normally on.The collector of transistor 137 rises, cutting off diode 144 and drivingtransistor 134 into hard conduction, transistor 134 being biased so asto be normally off. The resulting voltage drop at the collector oftransistor 134 reflexly further cuts off transistor 137 and cuts offdiode 136, making the latter unresponsive to any trailing edge of theinput pulse. Capacitor 139 then charges through resistor 138 and thecollector-emitter path of transistor 134 until the conduction thresholdof the base of transistor 137 is reached. Transistor 137 then reflexlyconducts and delivers a delayed negative pulse through diode 144 to thebase of transistor 145. The magnitude of the time delay is determined bythe values of capacitor 139 and resistor 138 which are selected toprovide a delay of 120 milliseconds.

The lowering of the potential on the base terminal of transistor 145turns it on which results in a positive-going pulse appearing on outputterminal 96. This pulse, it will be recalled, will turn on transistor 37in pulse generating means 10 in the absence of ventricular activity.Continued absence of natural R-waves in the heart will result in pulsessynchronized to the occurrence of P-waves in the atrium.

During synchronous operation, however, a pacer stimulating pulse willnot occur during the T-wave portion of the heart heat. This is becauseresistor 147 and Zener diode 148 comprise a lockout circuit and act tolimit the output of signal responsive means or P-wave detector 23 to apulse amplitude somewhat less than the amplitude required to fire pulsegenerator early in the recycle period of generator 10. Later in therecycle period, the voltage at the base of transistor 37 in FIG. 3 willhave risen to a point where the pulse amplitude of the output of P-wavedetector 23 is sufficient to produce a delayed R-wave.

In some circumstances it may be desired to disable the system or switchbetween the three pacer modes, i.e., nonsynchronous, demand andsynchronous, manually rather than automatically. A switch can beprovided to activate only pulse generating means 10 for continual,nonsynchronous operation. Having the switch set for activation of bothsignal generating means 10 and signal responsive means 19 would producea continual demand mode. Switching power to the generating means 10 andresponsive means 23 would provide a continual synchronous mode. Theswitch would have also a position whereby simultaneous or multi-modeoperation is rovided. A multiposition switch for accomplishing thisselective operation is indicated schematically at 150 in FIGS. 3-5, andfunctions, simply, to control selectively the application of power formthe supply to the three circuits l0, l9 and 23. Such a switch could bean R-F activated solid-state semiconductor, a magnetically activatedreed switch or switches, or can be percutaneously operated.

From the foregoing it is apparent that the present inventionaccomplishes its intended objects. An artificial pacemaker constructedin accordance with the present invention is responsive to the changingneeds of the body yet will not com pete under any circumstances withnatural cardiac electrical activity, normally conducted or ectopic.While a single specific embodiment of the invention has been described,this has been done by way of illustration, without thought oflimitation.

lclaim:

l. A cardiac pacer comprising:

a. pulse generating means including timing means controlling thegeneration of pulses;

b. first and second electrodes coupled to said pulse generating means,at least one of said electrodes adapted to be operatively connected to apatients heart on or in the ventricle thereof;

c. a third electrode adapted to be operatively connected to a patientsheart on or in the atrium thereof;

(1. first signal responsive means coupled to at least the ventricularone of said first and second electrodes and responsive to electricalsignals in the ventricle of the heart, said signal responsive meansbeing operatively connected to said pulse generating means so that aventricular electrical signal causes said signal responsive means toreset said timing means to a predetermined level;

e. second signal responsive means coupled to said third electrode andresponsive to electrical signals in the atrium of the heart, said secondsignal responsive means including timing means and being operativelyconnected to said pulse generating means so that each natural atrialbeat causes said second signal responsive means to activate said pulsegenerating means a predetermined time after the atrial beat andcoordinated with the natural beat;

f. said first signal responsive means including means for inhibitingactivation of said pulse generating means by said second signalresponsive means for a predetermined time following the occurrence of anatural ventricular beat; and

g. electrical power supply means connected to said pulse generatingmeans and to said first and second signal responsive means.

2. A totally implantable cardiac pacemaker according to claim 1 whichfurther includes moistureproof, human body reaction-free enclosure meansenveloping said pulse generating means, said first and second signalresponsive means, and said power supply means.

3. The cardiac pacer according to claim 1 wherein said pulse generatingmeans including timing means comprises:

a. a semiconductor pulse generator;

b. an RC timing circuit connected to said power supply and to said pulsegenerator for controlling the generation of pulses; and

c. a semiconductor switch connected to and controlled by said firstsignal responsive means and operatively connected to said R-C timingcircuit for preventing said R-C circuit from reaching a predeterminedvoltage level in response to electrical signals in the ventricle of theheart.

4. The cardiac pacer according to claim 3 wherein the output of saidsecond signal responsive means is connected to said R-C timing circuitfor controlling the generation of pulses.

5. The cardiac pacer according to claim 1 wherein said first signalresponsive means includes means responsive to cardiac electrical signalsof either polarity.

6. The cardiac pacer according to claim 5 wherein said first signalresponsive means comprises:

a. a plurality of semiconductor amplifier stages;

b. signal transmission means connected to the input of the first of saidsemiconductor amplifier stages;

c. means connected to the output of the last of said semiconductoramplifier stages for converting a monopolar input signal of eitherpolarity into a bipolar output signal; and v d. semiconductor switchingmean having a control terminal coupled to the output of said signalconverting means and connected between said power supply and said pulsegenerating means.

7. The cardiac pacer according to claim 6 wherein said signaltransmission means comprises a filter network adapted to reject signalshaving a frequency of around 60 cycles per second.

8. The cardiac pacer according to claim 6 wherein said signal convertingmeans comprises an R-C differentiating circuit.

9. The cardiac pacer according to claim 7 wherein the first of saidsemiconductor amplifier stages includes a field-effect transistor, thegate terminal of which is connected to the output of said filternetwork.

10. The cardiac pacer according to claim 1 wherein said second signalresponsive means comprises:

a. a plurality of semiconductor amplifier stages;

b. signal transmission means connected to the input of the first of saidsemiconductor amplifier stages;

c. means connected to the output of the last of said semiconductorstages for converting a monopolar input signal of either polarity into abipolar output signal;

d. trigger pulse producing means connected to the output of said signalconverting means;

e. a circuit connected to said trigger pulse producing means andoperative in response to the occurrence of a trigger pulse for providingan output pulse having a predetermined width after a predetermined timedelay; and

f. pulse amplitude limiting means connected to the output of saidcircuit.

11. The cardiac pacer according to claim 10 wherein said signaltransmission means comprises a filter network adapted to reject signalshaving a frequency of around 60 cycles per second.

12. The cardiac pacer according to claim 10 wherein said signalconverting means comprises an R-C differentiating circuit.

13. The cardiac pacer according to claim 10 wherein said circuitresponsive to a trigger pulse comprises:

a. a first semiconductor switch connected to the output thereof.

b. an R-C timing circuit connected to the input thereof and to thecontrol terminal of said first semiconductor switch; and

c. a second semiconductor switch connected to and controlled by saidfirst semiconductor switch and providing a conduction path for said R-Ccircuit.

14. The cardiac pacer according to claim llll wherein the first of saidsemiconductor amplifier stages includes a field effect transistor, thegate terminal of which is connected to the output of said filternetwork.

15. The cardiac pacer according to claim 1 further includingindependently operable switching means for selectively connecting saidelectrical power supply means to said pulse generating means and to saidfirst and second signal responsive means.

116. The cardiac pacer according to claim 6 further includ-

1. A cardiac pacer comprising: a. pulse generating means includingtiming means controlling the generation of pulses; b. first and secondelectrodes coupled to said pulse generating means, at least one of saidelectrodes adapted to be operatively connected to a patient''s heart onor in the ventricle thereof; c. a third electrode adapted to beoperatively connected to a patient''s heart on or in the atrium thereof;d. first signal responsive means coupled to at least the ventricular oneof said first and second electrodes and responsive to electrical signalsin the ventricle of the heart, said signal responsive means beingoperatively connected to said pulse generating means so that aventricular electrical signal causes said signal responsive means toreset said timing means to a predetermined level; e. second signalresponsive means coupled to said third electrode and responsive toelectrical signals in the atrium of the heart, said second signalresponsive means including timing means and being operatively connectedto said pulse generating means so that each natural atrial beat causessaid second signal responsive means to activate said pulse generatingmeans a predetermined time after the atrial beat and coordinated withthe natural beat; f. said first signal rEsponsive means including meansfor inhibiting activation of said pulse generating means by said secondsignal responsive means for a predetermined time following theaccurrence of a natural ventricular beat; and g. electrical power supplymeans connected to said pulse generating means and to said first andsecond signal responsive means.
 2. A totally implantable cardiacpacemaker according to claim 1 which further includes moistureproof,human body reaction-free enclosure means enveloping said pulsegenerating means, said first and second signal responsive means, andsaid power supply means.
 3. The cardiac pacer according to claim 1wherein said pulse generating means including timing means comprises: a.a semiconductor pulse generator; b. an R-C timing circuit connected tosaid power supply and to said pulse generator for controlling thegeneration of pulses; and c. a semiconductor switch connected to andcontrolled by said first signal responsive means and operativelyconnected to said R-C timing circuit for preventing said R-C circuitfrom reaching a predetermined voltage level in response to electricalsignals in the ventricle of the heart.
 4. The cardiac pacer according toclaim 3 wherein the output of said second signal responsive means isconnected to said R-C timing circuit for controlling the generation ofpulses.
 5. The cardiac pacer according to claim 1 wherein said firstsignal responsive means includes means responsive to cardiac electricalsignals of either polarity.
 6. The cardiac pacer according to claim 5wherein said first signal responsive means comprises: a. a plurality ofsemiconductor amplifier stages; b. signal transmission means connectedto the input of the first of said semiconductor amplifier stages; c.means connected to the output of the last of said semiconductoramplifier stages for converting a monopolar input signal of eitherpolarity into a bipolar output signal; and d. semiconductor switchingmean having a control terminal coupled to the output of said signalconverting means and connected between said power supply and said pulsegenerating means.
 7. The cardiac pacer according to claim 6 wherein saidsignal transmission means comprises a filter network adapted to rejectsignals having a frequency of around 60 cycles per second.
 8. Thecardiac pacer according to claim 6 wherein said signal converting meanscomprises an R-C differentiating circuit.
 9. The cardiac pacer accordingto claim 7 wherein the first of said semiconductor amplifier stagesincludes a field-effect transistor, the gate terminal of which isconnected to the output of said filter network.
 10. The cardiac paceraccording to claim 1 wherein said second signal responsive meanscomprises: a. a plurality of semiconductor amplifier stages; b. signaltransmission means connected to the input of the first of saidsemiconductor amplifier stages; c. means connected to the output of thelast of said semiconductor stages for converting a monopolar inputsignal of either polarity into a bipolar output signal; d. trigger pulseproducing means connected to the output of said signal converting means;e. a circuit connected to said trigger pulse producing means andoperative in response to the occurrence of a trigger pulse for providingan output pulse having a predetermined width after a predetermined timedelay; and f. pulse amplitude limiting means connected to the output ofsaid circuit.
 11. The cardiac pacer according to claim 10 wherein saidsignal transmission means comprises a filter network adapted to rejectsignals having a frequency of around 60 cycles per second.
 12. Thecardiac pacer according to claim 10 wherein said signal converting meanscomprises an R-C differentiating circuit.
 13. The cardiac paceraccording to claim 10 wherein said circuit responsive to a trigger pulsecomprises: a. a first semiconductor switch connected to the outputtHereof. b. an R-C timing circuit connected to the input thereof and tothe control terminal of said first semiconductor switch; and c. a secondsemiconductor switch connected to and controlled by said firstsemiconductor switch and providing a conduction path for said R-Ccircuit.
 14. The cardiac pacer according to claim 11 wherein the firstof said semiconductor amplifier stages includes a field effecttransistor, the gate terminal of which is connected to the output ofsaid filter network.
 15. The cardiac pacer according to claim 1 furtherincluding independently operable switching means for selectivelyconnecting said electrical power supply means to said pulse generatingmeans and to said first and second signal responsive means.
 16. Thecardiac pacer according to claim 6 further including a frequencyresponsive R-C network connected to the output of said semiconductorswitching means for rendering said switching means unresponsive tosignals above a predetermined rate.
 17. The cardiac pacer according toclaim 10 further including a frequency responsive R-C network connectedbetween said converting means and said trigger pulse producing means forrendering said trigger pulse producing means unresponsive to signalabove a predetermined rate.